This invention relates to a soffit ventilator. More particularly, it relates to a ventilator which is open to allow passage of air into a building at wind velocities below a predetermined value and closed to prevent air passage at velocities above such value. The predetermined wind velocity is that which is considered to generate an unreasonable risk of accumulating an unacceptable amount of wind-driven snow in the building. Soffit ventilators are well known and are considered to be particularly effective in minimizing heat buildup in spaces under a roof, particularly when used in combination with ridge ventilators. A problem which may arise in the use of a soffit ventilator, however, is the accumulation of wind-driven snow, rain or other particulate matter in the building. A number of factors may affect the amount of snow that might enter and accumulate in a building, such as exposure of the building to the wind, ratio of the soffit ventilator area to the ridge ventilator area, wind velocity, rate of snowfall, temperature and size of flakes, for example. It is apparent, however, that regardless of the various factors, one solution to the problem is to close the ventilator when wind-driven snow conditions are present. At least two patents describe soffit ventilators which may be closed. The ventilator in Frisby U.S. Pat. No. 2,755,728 has a frame which attaches to the soffit. The frame is covered by a screen to allow free passage of air therethrough while keeping insects and small animals out. The frame is also adapted to receive a manually applied snap-on cover when excessive winds occur. Shaklee U.S. Pat. No. 4,315,455 also shows a screened frame which attaches to the soffit and is adapted for closure. It is provided with a panel hinged to one side of the frame which can be moved from an open position to a closed position where it completely covers the screened opening. The panel and frame are adapted so that the panel can be maintained in an open or closed position or a number of intermediate positions therebetween. The Frisby and Shaklee patents describe ventilators which could be effective in preventing snow from accumulating in a building by closing the ventilators, but both of the described ventilators require manual handling of a cover to effect a closure. Devices having slotted or louvered openings which may be opened or closed automatically by moving air are known. Attic exhaust fans, for example, are typically mounted over a louvered opening in a ceiling separating an attic space from a space below. When the fan is turned on, a pressure differential is created between the attic side and lower side of the louvers causing them to open and remain open until the fan is turned off.